1. Technical Field
The invention generally relates to batteries, particularly to electrodes of lithium batteries.
2. Related Art
A lithium battery includes a positive electrode, a negative electrode and a separator between the two electrodes. A positive electrode is primarily made of an active material. The active material is mixed with conductive and binder to form a pulp. The pulp is spread on a collector which is a metallic film or net. In early years, LiCoO2 was a primary material serving as a positive active material because it was easy to be made and its performance was sufficient. However, the price of cobalt kept increasing because of its rarity, so that this material has not been disadvantageous to industrial application. Therefore, some new materials featuring high performance and low cost has appeared in the market such as LiNi1-x-yCoxMnyO2 and LiMn2O4.
However, these new materials also have respective drawbacks. For example, the conductivity of LiNi1-x-yCoxMnyO2 is relatively low and its safety is suspectful. LiMn2O4 is very cheap, only about one fifth of LiCoO2, but its capacity per gram is lower than LiCoO2 by 10% and it has a problem of manganese ion leakage.
Taiwan patent No. I233231 provides a method for manufacturing a positive active material with a coat of nano-oxide. '231 discloses a positive active material and a metal oxide which is coated thereon and whose size is about 10 nm-100 nm. The method of '231 includes the steps of: (1) adding a surface modifier and aqueous solution containing Mg(OH)2 into a precursor of positive material Co0.2Ni0.8(OH)2; (2) uniformly coating the precursor Co0.2Ni0.8(OH)2 with Mg(OH)2 by heating to remove water; (3) mixing LiOH and the precursor obtained in step (2), and then sintering the mixture for about 16 hours. The nano-metal oxide coated on the positive material may increase safety of material and have high capacity per gram due to a thin interface of the non-electricity-storing active area. Also, the cyclic life may be extended and large current charge/discharge may be greater than ever.
According to the above prior art, we can find out a fact that a composite material can indeed enhance performance of the positive material. However, some drawbacks are still remained, for example:
(1) Adding salt solution containing cobalt, aluminum, zinc or magnesium requires a dewatering process, so that it is disadvantageous to rapidly implement the whole manufacturing process.
(2) For the active materials such as LiCoO2, a great crystalline phase can be obtained only when a sintering process has been performed for a long time. However, the coating layer is hard to be formed because the coating material tends to diffuse into the coated material under the condition of high temperature and long time.